Many piping system applications in petro-chemical and other industries involve the handling of corrosive, erosive, scaling or otherwise hard-to-handle fluids. Piping materials that can withstand these fluids can be very costly. One economic approach to handling these difficult fluids is to cover or to line the interior of low cost (non-fluid-resistant) piping with a liner which is fluid-resistant. The low-cost pipe material, such as carbon steel, provides cost-effective structural support for the fluid resistant, but less structurally adequate liner. Even when a liner is composed of fluid resistant materials, more severe applications (such as handling erosive geothermal fluids) tend to erode, chip, spall, crack, pit, and delaminate the lining material, requiring thicker liners. Thin liners may also experience coverage and tool damage problems. One type of cost effective thick liner is composed of a fluid resistant, but brittle material, such as cement.
Lined-pipe connectors typically have a primary seal at a structural interface and a secondary liner seal at a liner interface to prevent fluid from contacting non-fluid-resistant piping materials. The added liner seal must also be reliable since exposure of the non-fluid-resistant pipe material to the harsh fluids can cause piping failure even if the primary seal does not leak.
Some connectors having significantly loaded liner gaskets or seals satisfy the need for a reliable liner seal, but significantly loaded liner seals may not be practical for fragile or brittle liners. In addition, liner sealing surface preparations needed (e.g., machining) can impose other unacceptable demands on the brittle liner, resulting in damage to the brittle liner and failure at the piping joint.
One type of soft elastomeric liner seal, such as an O-ring, also typically requires a groove or retaining edge to be provided in the liner end surface. In addition to loading and anchoring the elastomeric material, the groove can provide space for seal distortion isolated from the fluid stream flow.
However, this type of seal tends to require smoother sealing surfaces and tighter tolerances (e.g., on the groove depth) when compared to gasket type seals. But reliably obtaining these finishes and tolerances for a cast cement liner sealing surfaces may not be feasible, even if machined after casing. Grooves may also concentrate stresses in a brittle liner.
Creating a reliable liner end seal is particularly challenging when a threaded connector is used. The sealing element must be compressed while at the same time be able to accept relative rotation of the joint elements (e.g., during threaded joint assembly). Since typical soft elastomeric materials used for seals, such as synthetic rubbers, also tend to adhere to sealing surfaces and have a relatively high coefficient of friction without lubrication, rotating adhering surfaces without shredding, tearing, abrading, or otherwise damaging the soft elastomeric material or brittle liner can be difficult, especially when the liner surfaces are rough and unfinished.
None of the current or alternative approaches eliminates the problems of reliable brittle liner sealing without risking damage to the liner and/or the seal. Even if the seal and liner edges are undamaged, the reliability of sealing at these lined joints may be less than desired.